Optical filter and spectrometer including sub-wavelength double grating structure, and optical apparatus including the optical filter and spectrometer

ABSTRACT

An optical filter may include a first reflector and a second reflector. The first reflector may include a plurality of first gratings having a first sub-wavelength dimension and being arranged to recur at a first interval in a first direction. The second reflector may be spaced apart from the first reflector and include a plurality of second gratings having a second sub-wavelength dimension and arranged to recur at a second interval in a direction parallel to the first direction. The first reflector and the second reflector may include different materials or different geometric structures from each other. Accordingly, it is easy to adjust the transmission wavelength characteristics of the optical filter.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/926,362, filed on Mar. 20, 2018, which claims priority from KoreanPatent Application No. 10-2017-0037672, filed on Mar. 24, 2017, in theKorean Intellectual Property Office, the disclosure of which isincorporated herein in its entirety by reference.

BACKGROUND 1. Field

The present disclosure relates to optical filters and spectrometersincluding sub-wavelength double grating structures and opticalapparatuses including the optical filters and the spectrometers.

2. Description of the Related Art

Optical devices for changing the transmission, reflection, polarization,phase, intensity, path, etc. of incident light are used in variousoptical fields. Attempts have recently been made to create miniaturizedoptical devices that have various optical properties by using structuresof sub-wavelength dimensions.

Sub-wavelength structures may also be applied to spectrometers. Ingeneral, a resonance structure having a specific resonance wavelengthmay be achieved by separating two reflectors by a predetermineddistance. Distributed Bragg reflectors, in which material layers havingdifferent refractive indices are repeatedly stacked to a thickness of ¼of the wavelength, may be used as the two reflectors. In this case,since the number of layers stacked has to be increased in order toincrease reflectance and since a resonance wavelength is obtained byadjusting the distance between the reflectors, it is not easy to obtaina desired resonance wavelength with a miniaturized form factor.

SUMMARY

Provided are spectrometers having small volumes and excellent spectralperformance by using sub-wavelength double grating structures.

Additional aspects will be set forth in part in the description whichfollows and, in part, will be apparent from the description, or may belearned by practice of the presented example embodiments.

According to an aspect of an example embodiment, an optical filter mayinclude: a first reflector including a plurality of first gratingshaving a first sub-wavelength dimension and being arranged to recur at afirst interval in a first direction; and a second reflector spaced apartfrom the first reflector and including a plurality of second gratingshaving a second sub-wavelength dimension being arranged to recur at asecond interval in a second direction parallel to the first direction.The first reflector and the second reflector may include differentmaterials or different geometric structures from each other.

The first sub-wavelength dimension may be different from the secondsub-wavelength dimension or the first interval may be different from thesecond interval.

The first interval may be identical to the second interval. First widthsof first cross-sections of the plurality of first gratings perpendicularto a first longitudinal direction of the plurality of first gratings maybe different from second widths of second cross-sections of theplurality of second gratings perpendicular to second a longitudinaldirection of the plurality of second gratings.

The plurality of first gratings may include a first material having afirst refractive index and the plurality of second gratings may includea second material having a second refractive index different form thefirst refractive index.

The plurality of first gratings and the plurality of second gratings maybe arranged so that longitudinal directions of the plurality of firstgratings and the plurality of second gratings are parallel to eachother.

Cross-sections of the plurality of first gratings and the plurality ofsecond gratings in directions perpendicular to longitudinal directionsof the plurality of first gratings and the plurality of second gratingsmay have one of rectangular shapes, trapezoidal shapes, polygonalshapes, circular shapes, elliptical shapes, semi-circular shapes, andsemi-elliptical shapes.

The optical filter may further include: a substrate configured tosupport the plurality of first gratings and including a third materialhaving a third refractive index less than a first refractive index ofthe plurality of first gratings.

The optical filter may further include: a fourth material layer having afourth refractive index less than the first refractive index of theplurality of first gratings and configured to cover the plurality offirst gratings.

The optical filter may further include: a fifth material layer locatedon the fourth material layer, having a fifth refractive index less thana second refractive index of the plurality of second gratings, andconfigured to cover the plurality of second gratings.

The first refractive index of the plurality of first gratings and thesecond refractive index of the plurality of second gratings may beidentical.

The fourth material layer and the fifth material layer may include anidentical material.

According to an aspect of an example embodiment, a spectrometer mayinclude: a sensor substrate including a plurality of light detectionelements; and a plurality of optical filters arranged to respectivelycorrespond to the plurality of light detection elements, each opticalfilter of the plurality of optical filters having a transmissionwavelength band that is different from transmission wavelength bands ofother optical filters of the plurality of optical filters. Each of theplurality of optical filters may include: a first reflector including aplurality of first gratings having a first sub-wavelength dimension andbeing arranged to recur at a first interval in a first direction; and asecond reflector spaced apart from the first reflector and including aplurality of second gratings, the plurality of second gratings having asecond sub-wavelength dimension and being arranged to recur at a secondinterval in a second direction parallel to the first direction. Thefirst reflector and the second reflector may have different materials ordifferent geometric structures from each other.

Center wavelengths of transmission wavelength bands of the plurality ofoptical filters may be distributed in a predetermined wavelength band.

The plurality of first gratings included in the each of the plurality ofoptical filters may have a uniform thickness.

The plurality of second gratings included in the each of the pluralityof optical filters may have a uniform thickness.

Longitudinal directions of the plurality of first and second gratingsincluded in the plurality of optical filters may be parallel to oneanother.

The sensor substrate and the plurality of optical filters may bemonolithically formed.

The spectrometer may further include: a polarizer having a polarizationaxis parallel to the longitudinal directions so that polarized lightparallel to the longitudinal directions is incident on the plurality ofoptical filters.

The plurality of optical filters may include: a first group of opticalfilters whose gratings have a first longitudinal direction parallel to afirst direction; and a second group of optical filters whose gratingshave a second longitudinal direction perpendicular to the firstdirection.

At least one optical filter included in the first group of opticalfilters and at least one optical filter included in the second group ofoptical filters may have an identical transmission wavelength band.

According to an aspect of an example embodiment, an optical apparatusincludes: a light source configured to emit light to an object; thespectrometer located on an optical path of the light emitted by thelight source and reflected from the object; and an analyzer configuredto analyze at least one from among a physical property, a shape, aposition, and a movement of the object by analyzing the light detectedby the spectrometer.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects will become apparent and more readilyappreciated from the following description of the example embodiments,taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional view illustrating a structure of an opticalfilter according to an example embodiment;

FIG. 2 is a perspective view illustrating two reflectors included in theoptical filter according to an example embodiment, for explaining theprinciple of transmitting light of a specific wavelength;

FIG. 3 illustrates graphs showing transmission characteristics accordingto a change in variables related to an asymmetric shape of the opticalfilter according to an example embodiment;

FIG. 4 illustrates graphs showing transmission characteristics accordingto a change in variables related to an asymmetric shape of the opticalfilter according to another example embodiment;

FIG. 5 illustrates graphs showing transmission characteristics accordingto a change in variables related to an asymmetric shape of the opticalfilter according to another example embodiment;

FIG. 6 illustrates graphs showing the optical filter achievingtransmission spectra having small full widths at half maximum (FWHMs)and high transmittances with respect to various center wavelengthsaccording to an example embodiment;

FIG. 7 is a cross-sectional view illustrating a structure of aspectrometer according to an example embodiment;

FIG. 8 is a cross-sectional view illustrating a structure of aspectrometer according to another example embodiment;

FIG. 9 is a plan view illustrating a structure of a spectrometeraccording to another example embodiment;

FIG. 10 is a cross-sectional view taken along line A-A′ of FIG. 9; and

FIG. 11 is a block diagram illustrating a configuration of an opticalapparatus according to an example embodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to example embodiments, which areillustrated in the accompanying drawings. In the drawings, likereference numerals refer to like elements and sizes of elements may beexaggerated for clarity and convenience. In this regard, the presentembodiments may have different forms and should not be construed asbeing limited to the descriptions set forth herein.

It will be understood that when a component is referred to as being “on”another component, the component can be directly on the other componentor intervening components may be present thereon.

While such terms as “first,” “second,” etc. may be used to describevarious components, such components are not limited to the above terms.The above terms are used only to distinguish one component from another.In other words, terms such as “first,” “second,” etc. do not necessarilyimply order, preference, or importance.

As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well and vice versa, unless the contextclearly indicates otherwise. It will be further understood that theterms “comprises” and/or “comprising” used herein specify the presenceof stated features or components, but do not preclude the presence oraddition of one or more other features or components.

In addition, terms such as “unit,” “module,” or the like refer to unitsthat perform at least one function or operation, and the units may beimplemented as hardware (e.g., a circuit, a microchip, a processor,etc.), software, or as a combination of hardware and software.

Connecting lines, or connectors shown in the various figures presentedare intended to represent exemplary functional relationships and/orphysical or logical couplings between various elements. It should benoted that many alternative or additional functional relationships,physical connections or logical connections may be present in apractical device.

Expressions such as “at least one of,” when preceding a list ofelements, modify the entire list of elements and do not modify theindividual elements of the list.

FIG. 1 is a cross-sectional view illustrating a structure of an opticalfilter 100 according to an example embodiment. To explain the principleof transmitting light of a specific wavelength, FIG. 2 is provided toillustrate a perspective view of first and second reflectors RE1 and RE2provided in the optical filter 100 according to an example embodiment.

The optical filter 100 may include the first reflector RE1 and thesecond reflector RE2 that are spaced apart from each other. The firstreflector RE1 and the second reflector RE2 may include gratingstructures with sub-wavelength dimensions.

A plurality of first gratings GR1 constituting the first reflector RE1are periodically arranged with a first period Pi in one direction. Theterm “period” as used herein may refer to the regular distance orinterval at which a series of objects are repeatedly arranged. Thus, aperiodically arranged gratings may be spatially arranged atsubstantially regular intervals and those intervals may be uniform. Thedirection in which the plurality of first gratings GR1 are arranged maybe an x-direction. The first gratings GR1 may have stripe shapes and alongitudinal direction of the first gratings GR1 may be a y-direction.Cross-sections of the first gratings GR1 in a direction perpendicular tothe longitudinal direction may have rectangular shapes having a width w₁and a thickness (or height) t₁. A plurality of second gratings GR2constituting the second reflector RE2 are periodically arranged with asecond period P₂ in a direction parallel to the direction in which thefirst gratings GR1 are arranged. The second gratings GR2 may have stripeshapes running in a parallel direction to the stripe shapes of the firstgratings GR1. Cross-sections of the second gratings GR2 in the directionperpendicular to the longitudinal direction may have rectangular shapeshaving a width w₂ and a thickness t₂.

Shape dimensions w₁, w₂, t₁, t₂, p₁, and p₂ related to the firstgratings GR1 and the second gratings GR2 may have sub-wavelength values.The sub-wavelength refers to a length value that is less than anoperating wavelength and less than a center wavelength of a transmissionwavelength band of the optical filter 100, that is, a resonancewavelength λ_(c) of a Fabry-Perot resonator formed by the first andsecond reflectors RE1 and RE2.

The optical filter 100 according to an example embodiment may adjust atransmission wavelength band by using a sub-wavelength double gratingstructure and employ an asymmetric structure in order to more finelyadjust performance and a wavelength band.

The expression “asymmetric structure” may be used to indicate adifferent structure from a symmetric structure in which the firstreflector RE1 and the second reflector RE2 are identical, and the firstreflector RE1 and the second reflector RE2 in the optical filter 100according to an example embodiment are asymmetric with regard to opticalmaterials or geometric structures.

When the first reflector RE1 and the second reflector RE2 are asymmetricwith regard to the geometric structures, the first reflector RE1 and thesecond reflector RE2 are different from each other in at least one fromamong variables w₁, t₁, and p₁ of the first reflectors RE1 and variablesw₂, t₂, and p₂ of the second reflector RE2. In other words, the firstreflector RE1 and the second reflector RE2 may have a different width,thickness, and/or period from each other.

When the first reflector RE1 and the second reflector RE2 are asymmetricwith regard to optical materials, optical materials of the firstgratings GR1 of the first reflector RE1 may be different from opticalmaterials of the second gratings GR2 of the second reflector RE2. Theoptical materials may be expressed by a refractive index or anabsorption coefficient for light, and the following will be explainedbased on a refractive index.

Referring to FIG. 2, the first reflector RE1 and the second reflectorRE2 may form a Fabry-Perot resonator.

The Fabry-Perot resonator is formed by a cavity between the first andsecond reflectors RE1 and RE2 having a high reflectance. Light enteringa space between the first and second reflectors RE1 and RE2 mayreciprocate between the first and second reflectors RE1 and RE2 thatface each other, which results in constructive interference anddestructive interference. In this case, light of a wavelengthcorresponding to a resonance wavelength λ_(c) may satisfy a constructiveinterference condition and may pass through the Fabry-Perot resonator.Light λ_(an) of another wavelength band may not pass through theFabry-Perot resonator. The performance of the Fabry-Perot resonator isgenerally considered to be better when the Fabry-Perot resonator has asmaller bandwidth with respect to the resonance wavelength λ_(c)corresponding to a transmission spectrum. The performance of theFabry-Perot resonator may be defined via a quality (Q) factor or a fullwidth at half maximum (FWHM).

Since the optical filter 100 according to an example embodiment employsa sub-wavelength grating structure as a reflector of the Fabry-Perotresonator, the optical filter 100 may have a high reflectance and aminimized volume.

The resonance wavelength λ_(c) that passes through the Fabry-Perotresonator is determined by optical materials and geometric structures ofthe first reflector RE1 and the second reflector RE2. For example, theresonance wavelength λ_(c) and a waveform of the transmission spectrumare determined by refractive indices of the first gratings GR1 and thesecond gratings GR2, a refractive index of a surrounding material,variables w₁, w₂, t₁, t₂, p₁, and p₂ related to geometric structures ofthe first and second reflectors RE1 and RE2, and a distance to betweenthe first and second reflectors RE1 and RE2.

Although three first gratings GR1 are included in the first reflectorsRE1 and three second gratings GR2 are included in the second reflectorRE2 in FIG. 1, embodiments are not limited thereto and the number of thefirst gratings GR1 and the second gratings GR2 may be changed to fewerthan three or more than three gratings. For example, tens to hundreds offirst gratings GR1 and second gratings GR2 may be repeatedly arranged aslong as the first gratings GR1 and the second gratings GR2 have agrating regularity that is large enough to form the Fabry-Perotresonator.

The optical filter 100 according to an example embodiment may have ahigh degree of freedom in performance such as a desired wavelength bandand a desired bandwidth by using asymmetry of the first and secondreflectors RE1 and RE2 with regard to optical materials or geometricstructures. Accordingly, the optical filter 100 may be used as a narrowband-pass filter or may be applied to a spectrometer having excellentspectral performance in a wide wavelength band.

A detailed configuration of the optical filter 100 will now beexplained.

As shown in FIG. 1, the optical filter 100 may further include asubstrate 110 that supports the plurality of first gratings GR1. Thesubstrate 110 may include a third material having a third refractiveindex less than a refractive index of the first gratings GR1.

A fourth material layer 125 may be formed on the first gratings GR1. Thefourth material layer 125 may include a material having a refractiveindex less than a refractive index of the first gratings GR1. The fourthmaterial layer 125 may be formed to cover the plurality of firstgratings GR1 and may support the plurality of second gratings GR2. Afifth material layer 135 may be formed to cover the second gratings GR2.The fifth material layer 135 may include a material having a refractiveindex less than a refractive index of the second gratings GR2.

Any of various materials having a high refractive index may be used asmaterials of the first gratings GR1 and the second gratings GR2. Forexample, any one from among monocrystalline silicon, polycrystallinesilicon, amorphous silicon, titanium oxide (TiO₂), titanium nitride(TiN), silicon nitride (SiN), and a transparent conductive oxide (ITO)may be used as materials of the first gratings GR1 and the secondgratings GR2. Alternatively, a group III-V semiconductor compound suchas gallium arsenide (GaAs) or gallium phosphide (GaP) may be used asmaterials of the first gratings GR1 and the second gratings GR2.Alternatively, a metal or a metal oxide may be used as materials of thefirst gratings GR1 and the second gratings GR2.

The first gratings GR1 and the second gratings GR2 may include materialshaving different refractive indices. However, embodiments are notlimited thereto, and when geometric shapes of the first reflector RE1and the second reflector RE2 are different from each other, that is,when the first reflector RE1 and the second reflector RE2 are differentfrom each other in at least one from among the widths w₁ and w₂, thethicknesses t₁ and t₂, and the periods p₁ and p₂, the first gratings GR1and the second gratings GR2 may include materials having the samerefractive index.

Although the cross-sections of the first gratings GR1 and the secondgratings GR2 in the direction perpendicular to the longitudinaldirection have rectangular shapes, embodiments are not limited thereto,and the cross-sections may have any of various shapes such astrapezoidal shapes, polygonal shapes, circular shapes, ellipticalshapes, semi-circular shapes, or semi-elliptical shapes.

The fourth material layer 125 and the fifth material layer 135 mayinclude low refractive index materials having a refractive index lessthan a refractive index of the first gratings GR1 and the secondgratings GR2. For example, the fourth material layer 125 and the fifthmaterial layer 135 may include at least one from among silicon oxide(SiO₂), a polymer-based material (e.g., SU-8 or poly(methylmethacrylate) (PMMA)), and hydrogen silsesquioxane (HSQ). The fourthmaterial layer 125 and the fifth material layer 135 may include the samematerial.

Regarding the asymmetry in the geometric shapes, any of various variablecombinations of the first reflector RE1 and the second reflector RE2 maybe selected. For example, the thicknesses t₁ and t₂ of the firstgratings GR1 and the second gratings GR2 may be the same, the widths w₁and w₂ of the first gratings GR1 and the second gratings GR2 may bedifferent from each other, and the periods p₁ and p₂ of the firstgratings GR1 and the second gratings GR2 may be different from eachother. All of the variables may be different or two of the variables maybe the same and the remaining one of the variables may be different fromthe two previous variables.

The optical filter 100 employing an asymmetric structure may more easilyobtain a wide wavelength band, adjust a position of a wavelength band,and have an FWHM than an optical filter having a symmetric structure,and may easily consider the ease of a process.

The optical filter 100 may be applied as a narrow band-pass filter tovarious optical apparatuses. For example, a color filter may be realizedby forming a plurality of optical filters having red, green, and bluewavelengths as transmission bands and repeatedly arranging the pluralityof optical filters. The color filter may have high color purity and maybe applied to various types of display devices.

FIG. 3 illustrates graphs showing transmission characteristics accordingto a change in variables related to an asymmetric shape of the opticalfilter 100 according to an example embodiment.

The graphs are transmission spectra obtained when the periods p₁ and p₂are set to be the same, the thicknesses t₁ and t₂ are set to be thesame, the widths w₁ and w₂ are set to be different from each other, thewidth w₂ is fixed at 270 nm, and the width w₁ varies between 150 nm and400 nm.

It is found that various types of transmission spectra having centerwavelengths ranging from about 825 nm to about 845 nm may be obtained asvariables are adjusted. A transmission spectrum having a hightransmittance and a small FWHM (or a high Q factor) may be selectedaccording to needs from among the transmission spectra.

FIG. 4 illustrates graphs showing transmission characteristics accordingto a change in variables related to an asymmetric shape of the opticalfilter 100 according to another example embodiment.

The graphs are transmission spectra obtained when the periods p₁ and p₂are set to be the same but are different from those in FIG. 3, thethicknesses t₁ and t₂ are set to be the same, the widths w₁ and w₂ areset to be different from each other, the width w₂ is fixed at 260 nm,and the width w₁ varies between 235 nm and 295 nm.

It is found that various types of transmission spectra having centerwavelengths ranging from about 840 nm to about 855 nm may be obtained asvariables are adjusted. A transmission spectrum having a hightransmittance and a small FWHM may be selected according to needs fromthe transmission spectra.

FIG. 5 illustrates graphs showing transmission characteristics accordingto a change in variables related to an asymmetric shape of the opticalfilter 100 according to another example embodiment.

The graphs are transmission spectra obtained when the periods p₁ and p₂are set to be the same but are different from those in FIGS. 3 and 4,the thicknesses t₁ and t₂ are set to be the same, the widths w₁ and w₂are set to be different from each other, the width w₂ is fixed at 260nm, and the width w₁ varies between 150 nm and 400 nm.

It is found that various types of transmission spectra having centerwavelengths ranging from about 855 nm to about 875 nm may be obtained asvariables are adjusted. A transmission spectrum having a hightransmittance and a small FWHM may be selected according to needs fromthe transmission spectra.

FIG. 6 illustrates graphs showing the optical filter 100 achievingtransmission spectra having small FWHMs and high transmittances withrespect to various center wavelengths according to an exampleembodiment.

The graphs are obtained by selecting transmission spectra having FWHMsof about 1 nm or less and transmittances close to 1 with respect toseveral center wavelengths in the graphs of FIGS. 3 through 5. It isfound that transmission spectra having small FWHMs and centerwavelengths that are uniformly distributed between about 830 nm andabout 870 nm and FWHMs may be achieved.

The above graphs are exemplary, and transmission spectra having smallFWHMs and center wavelengths that are uniformly distributed in variouswavelength bands as well as the above wavelength bands may be achieved.A spectrometer having excellent spectral performance may be achieved byusing various asymmetric shapes of the optical filter 100 related to thetransmission spectra.

FIG. 7 is a cross-sectional view illustrating a structure of aspectrometer 300 according to an example embodiment.

The spectrometer 300 may include a sensor substrate 210 including aplurality of light detection elements 212, and a plurality of opticalfilters 250_k (where k=1, . . . , and n) arranged to respectivelycorrespond to the plurality of light detection elements 212 and havingdifferent transmission wavelength bands. For example, photodiodes,phototransistors, or charge-coupled devices (CODs) may be used as theplurality of light detection elements 212. The number n of the opticalfilters 250_k may be appropriately determined according to the use ofthe spectrometer 300 in consideration of a wavelength band included inthe light which is to be separated.

Each of the plurality of optical filters 250_k includes, like theoptical filter 100 of FIG. 1, a first reflector RE_(k1) including aplurality of first gratings GR_(k1), which have sub-wavelength shapedimensions and are periodically arranged, and a second reflector RE_(k2)spaced apart from the first reflector RE_(k1) and including a pluralityof second gratings GR_(k2), which have sub-wavelength shape dimensionsand are periodically arranged.

Each of the plurality of optical filters 250_k is chosen so that thefirst reflector RE_(k1) and the second reflector RE_(k2) have differentmaterials or different geometric structures and a transmission spectrumhaving a center wavelength λ_(k) is achieved.

The optical filter 250_1 may a first reflector RE₁₁ and a secondreflector RE₁₂, and thicknesses t₁₁ and t₁₂, periods p₁₁ and p₁₂, andwidths w₁₁ and w₁₂ of first and second gratings GR₁₁ and GR₁₂, and adistance t₁₀ between the first reflector RE₁₁ and the second reflectorRE₁₂ may be selected such that a transmission spectrum having a centerwavelength λ₁ is obtained. The thicknesses t₁₁ and t₁₂, the periods p₁₁and p₁₂, and the widths w₁₁ and w₁₂ of the first and second gratingsGR₁₁ and GR₁₂ have values less than the center wavelength λ₁.

The optical filter 250_k may include a first reflector RE_(k1) and asecond reflector RE_(k2), and thicknesses t_(k1) and t_(k2), periodsp_(k1) and p_(k2), and widths w_(k1) and w_(k2) of first and secondgratings GR_(k1) and GR_(k2,) and a distance t_(k0) between the firstreflector RE_(k1) and the second reflector RE_(k2) may be selected suchthat a transmission spectrum having a center wavelength λ_(k) isobtained. The thicknesses t_(k1) and t_(k2), the periods p_(k1) andp_(k2), and the widths w_(k1) and w_(k2) of the first and secondgratings GR_(k1) and GR_(k2) have values less than the center wavelengthλ_(k).

The optical filter 250_n may include a first reflector RE_(n1) and asecond reflector RE_(n2), and thicknesses t_(n1) and t_(n2), periodsp_(n1) and p_(n2), and widths w_(n1) and w_(n2) of first and secondgratings GR_(n1) and GR_(n2), and a distance t_(n0) between the firstreflector RE_(n1) and the second reflector RE_(n2) may be selected suchthat a transmission spectrum having a center wavelength λ_(n) isobtained. The thicknesses t_(n1) and t_(n2), the periods p_(n1) andp_(n2), and the widths w_(n1) and w_(n2) of the first and secondgratings GR_(n1) and GR_(n2) have values less than the center wavelengthλ_(n).

Longitudinal directions of the first and second gratings GR_(k1) andGR_(k2)provided in the plurality of optical filters 250_k may beparallel to one another and may be, for example, a y-direction.

As described with reference to FIGS. 3 through 6, the variables t_(k1),t_(k2), p_(k2), w_(k1), w_(k2,) and t_(k0) of the optical filters 250_kmay be adjusted so that the spectrometer 200 covers a predeterminedrange of wavelengths and center wavelengths of transmission wavelengthbands of the optical filters 250_k are uniformly distributed in thepredetermined range of wavelengths.

The sensor substrate 210 and the plurality of optical filters 250_k maybe monolithically formed. That is, the plurality of first gratingsGR_(k1) may be directly formed on the sensor substrate 210, and a firstmaterial layer 225, the second gratings GR_(k2), and a second materiallayer 235 may be sequentially formed.

For the ease of a manufacturing process, the thicknesses t_(k1) of thefirst gratings GR_(k1) included in the plurality of optical filters250_k may be the same and the distances t_(k0) between the firstreflectors RE_(k1) and the second reflectors RE_(k2) of the plurality ofoptical filters 250_k may also be the same. This is because it may beeasier to form the plurality of second gratings GR_(k2) on the sameplane of the first material layer 225. Likewise, the thicknesses t_(k2)of the second gratings GR_(k2) formed on the first material layer 225may also be the same.

In this structure, major variables related to transmissioncharacteristics of each optical filter 250_k may be w_(k1), w_(k2),p_(k1), and p_(k2), and an appropriate combination of the variables maybe set in consideration of achieving an FWHM and the center wavelengthλ_(k).

FIG. 8 is a cross-sectional view illustrating a structure of aspectrometer 201 according to another example embodiment.

The spectrometer 201 is different from the spectrometer 200 of FIG. 7 inthat the spectrometer 201 further includes a polarizer 270. Thepolarizer 270 may have a polarization axis parallel to a longitudinaldirection of the first and second gratings GR_(k1) and GR_(k2). That is,the polarizer 270 may allow only polarized light parallel to thelongitudinal direction to be transmitted therethrough and be incident onthe plurality of optical filters 250_k. Light to be separated incidenton the spectrometer 201 may include various pieces of polarized light.Spectral performance, that is, a transmittance of a transmissionwavelength band achieved by each optical filter 250_k, is the highestwhen polarization direction is parallel to the longitudinal direction ofthe first and second gratings GR_(k1) and GR_(k2) included in theoptical filter 250_k. Accordingly, spectral performance may be improvedwith the addition of the polarizer 270 having the polarization axis. Asshown in FIG. 8, the polarizer 270 may be directly adhered to the secondmaterial layer 235. However, embodiments are not limited thereto, andthe polarizer 270 may be located at any position in an optical paththrough which light to be separated travels to an array of the opticalfilters 250_k.

Since the polarizer 270 allows only polarized light parallel to thepolarization axis from among pieces of polarized light to be transmittedtherethrough, the amount of light incident on the optical filters 250_kmay be reduced, thereby reducing efficiency.

FIG. 9 is a plan view illustrating a structure of a spectrometer 300according to another example embodiment. FIG. 10 is a cross-sectionalview taken along line A-A′ of FIG. 9.

The spectrometer 300 may include a first group 351 and a second group352 that are divided according to longitudinal directions of thegratings GR_(k1) and GR_(k2) included in optical filters 351_k and 352_k(where k=1, . . . , and n). The first group 351 may include theplurality of optical filters 351_k, and the longitudinal direction ofthe gratings GR_(k1) and GR_(k2) included in the optical filters 351_kmay be a first direction, for example, a y-direction. The second group352 may include the plurality of optical filters 352_k, and thelongitudinal direction of the gratings GR_(k1) and GR_(k2) included inthe optical filters 352_k may be a second direction, for example, anx-direction, that is perpendicular to the first direction.

A wavelength band covered by the plurality of optical filters 351_kincluded in the first group 351 and a wavelength band covered by theplurality of optical filters 352_k included in the second group 352 maybe set to be the same or similar to each other. At least one from amongthe optical filters 351_k included in the first group 351 and at leastone from among the optical filters 352_k included in the second group352 may have the same transmission wavelength band. Although all ofwavelengths denoted by λ₁ through λ_(n) are included in each group inFIG. 9, embodiments are not limited thereto.

As shown in FIG. 10, a sensor substrate 310 in which a plurality oflight detection elements 312 are formed and the plurality opticalfilters 351_k and 352_k may be monolithically formed. That is, theplurality of first gratings GR_(k1) may be directly formed on the sensorsubstrate 310, and a fourth material layer 325, the second gratingsGR_(k2), and a fifth material layer 335 may be sequentially formed.

Since the spectrometer 300 having this structure includes one set ofgratings GR_(k1) and GR_(k2) whose longitudinal directions areperpendicular to each other, the spectrometer 300 does not include anadditional polarizer. Even when light to be separated includes variouspieces of polarized light, the spectrometer 300 may have desiredspectral performance without reducing spectral efficiency. However,embodiments are not limited thereto, and in another example embodiment,an additional polarizer having a polarization axis parallel to thelongitudinal direction of each of the gratings GR_(k1) and GR_(k2) maybe further provided on each of the plurality of optical filters 351_kand 352_k to improve spectral performance of each polarized light.

The structure of FIG. 8 or the structure of FIG. 9 may be selectedconsidering a beam diameter of light to be separated.

The above-described spectrometer may be applied to various opticalapparatuses and sensors. For example, the spectrometer may be applied toa gas sensor or a chemical sensor. The sensor may recognize types ofvarious molecules present in the air and detect a density thereof byusing the spectrometer. In this case, the sensor may use the fact that atransmittance varies according to a wavelength due to a type and adensity of a component.

Also, the spectrometer may be used as a device for examining an object.For example, the spectrometer may be used as a device for analyzing aposition or a shape of an object or analyzing a component or a physicalproperty of an object according to Raman spectroscopy.

FIG. 11 is a block diagram illustrating a configuration of an opticalapparatus 1000 according to an example embodiment.

The optical apparatus 100 includes a light source 1200 configured toemit light to an object OBJ, a spectrometer 1500 located on an opticalpath of light emitted by the light source 1200 and reflected from orscattered from or transmitted into the object OBJ, and an analyzer 1700configured to analyze at least one from among a physical property, ashape, a position, and a movement of the object OBJ by analyzing lightdetected by the spectrometer 1500.

The spectrometer 1500 may include an optical filter array 1510 and alight detection element array 1530. The optical filter array 1510 mayinclude a plurality of optical filters using an asymmetric doublegrating structure as described above. Accordingly, excellent spectralperformance may be achieved, that is, center wavelengths may beuniformly distributed in a desired wavelength band, a transmittance maybe high, and an FWHM may be small.

An operation of the optical apparatus 1000 will now be explained byusing Raman spectroscopy.

Raman spectroscopy uses a phenomenon in which when light of a singlewavelength is scattered by interacting with molecular vibrations of amaterial of the object OBJ, energy state is shifted.

Light Li emitted by the light source 1200 may act as exciting light forthe object OBJ. The light source 1200 may emit short-wavelength lightsuitable to detect a wavelength shift. For example, the light source1200 may emit short-wavelength laser light in the form of pulses. Lightis scattered due to a molecular structure in the object OBJ. Light Lroutput from the object OBJ is scattered light that has a convertedwavelength due to the molecular structure in the object OBJ, and thescattered light may include various spectra having different degrees ofwavelength conversion according to molecular states in the object OBJ,which is referred to as a Raman signal.

When the Raman signal is incident on the spectrometer 1500, each of theoptical filters constituting the optical filter array 1510 may transmitlight having a wavelength corresponding thereto, the transmitted lightmay be incident on a light detection element of the light detectionelement array 1530, and an energy level of the transmitted light may bedetected.

The detected Raman signal is analyzed by the analyzer 1700. The Ramansignal may include information about a wavelength shift that occurs froma wavelength of incident light, and may include information related tomolecular vibrations of a material as an energy shift, for example,information about a molecular structure or a bonding type andinformation about a functional group. A Raman peak in a Raman spectrummay vary according to a molecular component of the object OBJ, and forexample, whether glucose, urea, ceramide, keratin, or collagen includedin blood or intercellular fluid of the object OBJ is included may beanalyzed. As such, the analyzer 1700 may analyze a material component, adensity, and a distribution amount in the object OBJ from the light fromthe object OBJ, that is, the Raman signal. The analyzer 1700 may beimplemented with software, hardware (e.g., a circuit, a microchip, aprocessor, etc.), or a combination of both software and hardware.

The optical apparatus 1000 may be used as a three-dimensional (3D)optical sensor, that is, an apparatus for sensing a shape and anoperation of the object OBJ, which will now be explained as follows.

The light source 1200 may emit the light Li including a plurality ofwavelength bands. The light Li may be emitted to scan the object OBJ. Tothis end, an optical element such as a beam steering device may befurther located between the light source 1200 and the object OBJ.

The light Lr from the object OBJ is received by the spectrometer 1500.The spectrometer 1500 may include the optical filter array 1510configured to transmit light to detect the light including the pluralityof wavelength bands emitted by the light source 1200.

The analyzer 1700 may analyze information about the object OBJ from asignal of the light including the plurality of wavelengths detected bythe spectrometer 1510. For example, the analyzer 1700 may determine a 3Dshape of the object OBJ by performing an operation for measuring atime-of-flight from the detected optical signal. Alternatively, theanalyzer 1700 may determine a shape of the object OBJ by directlymeasuring a time or performing an operation using correlation.

When the light source 1200 emits a plurality of pieces of light havingdifferent wavelengths and the spectrometer 1500 may detect the light Lrfrom the object OBJ according to wavelengths, for example, a speed atwhich the object OBJ is scanned may be increased and information about aposition and a shape of the object OBJ may be obtained at a relativelyhigh speed.

Although the optical apparatus 1000 analyzes a position and a shape ofthe object OBJ by analyzing light from the object OBJ or analyzing atype, a component, a density, and a physical property of the object OBJby using Raman spectroscopy that detects a wavelength shift due to theobject OBJ, embodiments are not limited thereto.

The optical apparatus 1000 may also include a controller for controllingan overall operation of the optical apparatus 1000, and may include amemory in which programs and other data needed to perform an operationof the analyzer 1700 are stored. The controller may be implemented withhardware (e.g., a circuit, a microchip, a processor, etc.), software, ora combination of both hardware and software. Further, the controller andthe analyzer 1700 may be integrated into one component.

An operation result of the analyzer 1700, that is, information about ashape, a position, and a physical property of the object OBJ, may betransmitted to another unit. For example, the information may betransmitted to a medical device using information about a property ofthe object OBJ, for example, biometric information, or an autonomousdevice requiring information about a 3D shape, a movement, and aposition of the object OBJ. Alternatively, the unit to which theinformation is transmitted may be a display device or a printer thatoutputs a result. Alternatively, the unit may be, but is not limited to,a smartphone, a mobile phone, a personal digital assistant (PDA), alaptop, a personal computer (PC), a wearable computing device, or amobile or non-mobile computing device.

Since the above-described optical filter has a high degree of freedom inachieving a transmission wavelength band and an FWHM by using anasymmetric double grating structure, the optical filter may provideexcellent transmittance characteristics at various center wavelengths.

The optical filter may be applied to a spectrometer, may have a smallstructure and excellent spectral performance, and may be applied tovarious optical apparatuses.

While the present disclosure has been particularly shown and describedwith reference to example embodiments thereof by using specific terms,the embodiments and terms have merely been used to explain the presentdisclosure and should not be construed as limiting the scope of thepresent disclosure as defined by the claims. The example embodimentsshould be considered in a descriptive sense only and not for purposes oflimitation. Therefore, the scope of the present disclosure is definednot by the detailed description of the present disclosure but by theappended claims, and all differences within the scope will be construedas being included in the present disclosure.

What is claimed is:
 1. An optical filter array comprising: a pluralityof optical filters including a first optical filter and a second opticalfilter, wherein each of the first optical filter and the second opticalfilter comprises: a first reflector comprising a plurality of firstnon-metallic gratings and a first material covering the plurality offirst non-metallic gratings and having a lower refractive index than arefractive index of the plurality of first non-metallic gratings, thefirst non-metallic gratings having a first sub-wavelength dimension andbeing arranged to recur at a first constant interval in a firstdirection; and a second reflector stacked on and spaced apart from thefirst reflector, the second reflector comprising a plurality of secondnon-metallic gratings and a second material covering the plurality ofsecond non-metallic gratings having a lower refractive index than arefractive index of the plurality of second non-metallic gratings, andthe plurality of second non-metallic gratings having a secondsub-wavelength dimension and being arranged to recur at a secondconstant interval in a second direction parallel to the first direction,wherein the first reflector and the second reflector have at least oneof different materials and different geometric structures from eachother, wherein the first optical filter and the second optical filterare arranged horizontally perpendicular to a stacking direction of thefirst reflector and the second reflector, and wherein the first opticalfilter and the second optical filter are configured to represent atransmission wavelength band that is different from each other.
 2. Theoptical filter array of claim 1, wherein, as to at least one of thefirst optical filter and the second optical filter, the first constantinterval is different from the second constant interval, and firstwidths of first cross-sections of the plurality of first gratings in athird direction perpendicular to a first longitudinal direction of theplurality of first gratings are different from second widths of secondcross-sections of the plurality of second gratings in a fourth directionperpendicular to a second longitudinal direction of the plurality ofsecond gratings.
 3. The optical filter array of claim 1, wherein adifference between a center wavelength of the transmission wavelengthband of the first optical filter and a center wavelength of thetransmission wavelength band of the second optical filter is smallerthan 10 nm.
 4. The optical filter array of claim 1, wherein a full widthat half maximum of a transmission spectrum of each the first opticalfilter and the second optical filter is smaller than 2 nm.
 5. Theoptical filter array of claim 1, wherein the transmission wavelengthband is from 820 nm to 880 nm.
 6. The optical filter array of claim 1,wherein the transmission wavelength band is from 820 nm to 880 nm. 7.The optical filter array of claim 2, wherein the first cross-sections ofthe plurality of first gratings in the third direction and the secondcross-sections of the plurality of second gratings in the fourthdirection have one of rectangular shapes, trapezoidal shapes, polygonalshapes, circular shapes, elliptical shapes, semi-circular shapes, andsemi-elliptical shapes.
 8. The optical filter array of claim 1, furthercomprising: a third optical filter arranged horizontally perpendicularto the stacking direction and are configured to represent differenttransmission wavelength band than the first optical filter and thesecond optical filter.
 9. The optical filter array of claim 8, whereinthe center wavelength of the transmission wavelength band of the firstoptical filter, the center wavelength of the transmission wavelengthband of the second optical filter, and a center wavelength of thetransmission wavelength band of the third optical filter are uniformlydistributed in a range from 820 nm to 880 nm.
 10. A spectrometercomprising: a sensor substrate comprising a plurality of light detectionelements; and an optical filter array of claim 1, the plurality ofoptical filters arranged to respectively correspond to the plurality oflight detection elements.
 11. The spectrometer of claim 10, wherein theplurality of first gratings included in the each of the plurality ofoptical filters have a uniform thickness.
 12. The spectrometer of claim10, wherein the plurality of second gratings included in the each of theplurality of optical filters have a uniform thickness.
 13. Thespectrometer of claim 10, wherein the sensor substrate and the pluralityof optical filters are monolithically formed.
 14. A smart phonecomprising the spectrometer of claim
 10. 15. An optical apparatuscomprising: a light source configured to emit light to an object; aspectrometer of claim 10, the spectrometer being located on an opticalpath of the light emitted by the light source and reflected from theobject; and an analyzer configured to analyze at least one from among aphysical property, a shape, a position, and a movement of the object byanalyzing the light detected by the spectrometer.
 16. A smart phonecomprising the optical apparatus of claim 15.